feat: Docker multi-stage build and CI/CD push

Improve Docker to multi-stage build and push to registry.

• Update central pipeline reference to main.

.gitlab-ci.yml

+# Comment to trigger build
+
 variables:
   CONTAINER_IMAGE: "graphpolaris/frontend"
-  IMAGE_TAG: "0.0.1-demo.0"
 
 include:
   - project: 'graphpolaris/pipelines'
     ref: fix-build-push
-    file: '.gitlab-ci.yml'
+    file: 'docker-build-push.yml'